The objectives of this protocol are to demonstrate the circadian rhythmicity of serum phosphorus in normal children and in their siblings with X-linked hypophosphatemic rickets during periods with and without phosphorus supplementation. At the same time, the metabolic clearance rate and production rate of calcitriol will be measured. Lastly, the effect of secondary hyperparathyroidism on the same measures will be determined in the patient population and in their normal siblings as controls.